Quote:
Originally Posted by wejones
Before getting into the "smaller declination", intent in alignment is to have the motor axis pretty close (not exactly) to being parallel to the earth's rotation axis, ie pointed at the north star (this is done with the motor elevation adjustment). Without declination, the dish would look out in a plane parallel to the earth's equatorial plane, but several thousand miles above it (ie the "A" distance). In order to point at the satellites, the dish has to be lowered by the declination angle, otherwise it would be pointed above all the sats.
The problem is, that (what I was trying to show with drawings) because we are different distances away from the different satellites, the necessary declination angle will be different for each sat. The maximum declination angle is for the sat to your south {5.99 deg in your case), and sats to your east or west will have a smaller declination angle {about 5.31 in your case}, and the sats in between are in between these two values. A properly aligned system should use the "smaller" {5.31} declination. This will require using a motor elevation setting which is less by the difference in these two numbers, ie by using a latitude setting that is greater than your actual latitude by 0.68 in this case, ie 38.269 instead of 37.5846.
For a small dish on Ku or a big dish on C band, it is probably close enough to just use the south declination {5.99}, and you will probably get acceptable alignment, however if you want to be more precise, it's best to try to start at the desired value, which is the 5.31.
With these small dishes, however, the declination setting (which is accomplished by the dish elevation) is only a starting point. Most people first set the motor elevation, then find the dish elevation by peaking on your south satellite, and if your motor elevation is set at your actual latitude, it will automatically peak on your south declination. So basically, the only thing you need to do is set your motor elevation to 38.269 (or as close to 38.3 as you can), and when you peak on your south sat, it will automatically set your declination to the proper value. No calculations necessary, just set your motor's latitude setting about 0.7 degrees higher than your actual latitude. That's all. |
I get it now. Thanks for the extra time going into so much detail. I have a few more questions though,
. I downloaded some apps from your page, namely the Sat View app. I did some calculations with it. At my latitude(I used 37) my max declination is 5.91. I subtracted 61.5(My most easterly sat) from 121(my most southerly sat), and came up with a delta longitude of 59.5, rounded it to 60. That makes my minimum declination 5.53. Would that be correct? So now I take my max declination 5.91 and subtract my minimum declination of 5.53, and I come up with .38. so I take my actual latitude(37.5846), and add .38, and I come with 37.9646, or rounded to 38.
Does that sound about right? I don't expect to use any sats farther east than Echo 3 at 61.5. I am getting mixed results on which is actually the furthest east sat I can get. One program says I can get PAS 6B, and PAS 3R at 43W, and another says the furthest I can get is PAS 1R at 45W. The programing on the sats further east are of no interest to me anyways, so it is a moot point.